1. Field of the Invention
This invention relates generally to methods and apparatus for measuring the operating characteristics of a rotating member. More particularly, this invention relates to methods and apparatus for sensing and displaying torsional vibrations associated with a drill string.
2. Description of the Related Art
During drilling operations, a drill string is subjected to axial, lateral, and torsional loads stemming from a variety of sources. In the context of a rotating drill string, torsional loads are imparted to the drill string by the rotary table, which rotates the drill string, and by the interference between the drill string and the wellbore. Axial loads act on the drill string as a result of the successive impacts of the drill bit on the cutting face, and as a result of irregular vertical feed rate of the drill string by the driller. The result of this multitude of forces applied to the drill string is a plurality of vibrations introduced into the drill string. The particular mode of vibration will depend on the type of load applied. For example, variations in the torque applied to the drill string will result in a torsional vibration in the drill string.
At the surface, torsional vibration in the drill string appears as a regular, periodic cycling of the rotary table torque. The torsional oscillations usually occur at a frequency that is close to a fundamental torsional mode of the drill string, which depends primarily on the drill pipe length and size, and the mass of the bottom hole assembly (BHA). The amplitude of the torsional vibrations depends upon the nature of the frictional torque applied to the drill string downhole, as well as the properties of the rotary table. Torsional vibrations propagating in a drill string are significant in that they are ordinarily accompanied by alternating acceleration and deceleration of the BHA and bit, as well as repeated twisting of the drill pipe section of the drill string.
Relatively high magnitudes of torsional vibrations are produced in the drill string by what is known in the industry as stick-slip behavior of the bit and BHA. At the outset of a stick-slip event, the rotation of the drill string grinds to a halt and the rotary table begins to stall due to the excessive buildup of torque in the drill string. As the rotary table continues to rotate, torque builds in the drill string as it twists. If the rotary table has sufficient torque capacity to overcome the resistance to rotation of the bit and BHA, the drill string will release and rotate in an uncontrolled condition, up to some maximum speed that is a function of the friction acting on the entire length of the drill string. The buildup of torque causes the drill string to shorten, while the uncontrolled rotation tends to lengthen the drill string.
The stick-slip phenomena represents the extreme outcome of high magnitude torsional vibrations. However, virtually all periodic variations in the torque applied to a drill string will cause uncontrolled accelerations of the various components making up the drill string, including the rotary table, bit, and BHA. Like a stick-slip phenomena, these uncontrolled accelerations of the drill string may cause damage to the bit, any tools in the BHA, and may also cause wiping damage to the walls of the wellbore.
There are a number of physical mechanisms associated with torsional vibration that may be damaging to the bit and drill string. First, torsional vibrations introduce cyclic stresses that accompany the non-uniform rotational motion of the drill string. Persistent twisting and unwinding of the drill string can lead to stalling of the drill string and overloading of connections, while fluctuating bit speed may cause fatigue failure of the cutting elements on the bit. Second, bursts of lateral BHA vibration can accompany the rotational accelerations of the BHA. These bursts can produce bending stresses that can ultimately cause connection failure or impact damage to sensitive components in the BHA, such as measurement while drilling (MWD) tools. Third, intermittent periods of backward BHA/Bit rotation may occur during stick-slip conditions, a phenomena that may lead to twist-offs. Finally, lengthening of the drill string due to uncontrolled forward rotation may drive the bit into the cutting face, leading to a stalled condition and a renewed stick-slip cycle.
The undesirable character of the physical phenomena associated with torsional vibrations are amplified by the relative ease with which torsional vibrations can be initiated in a drill string, the persistence of such torsional vibrations once initiated, and the minimal damping associated with low frequency forms of torsional vibration in a drill string. Most drill strings have a relatively low torsional stiffness. As a consequence, small torque fluctuations downhole can produce large rotational displacement. Once initiated, the torsional waves propagating along the drill pipe are reflected downhole by the relatively high impedance of the rotary table, and a self-perpetuating transfer of energy is created between the drill pipe and BHA sections that can build into full stick-slip behavior which may persist until conditions are changed.
If the driller is able to perceive the magnitude of the torsional vibrations active in the drill string, steps may be taken to alleviate a potentially damaging situation, such as increasing the rotary table speed or decreasing the weight on the bit (WOB). However, traditional drilling rig systems supply the driller with four basic parameters. Rotary table torque, rotary table speed, hook load, and stand pipe pressure. Although the readings from a typical meter showing rotary table torque will vary sinusoidally during stick-slip conditions, these analog meters merely represent a snapshot picture of the applied torque at a given instant, and cannot depict torsional vibrations usefully.
Previous attempts to sense and analyze torsional vibrations in a drill string have commonly centered on directly sensing vibrations using accelerometers encased in a measurement sub in the BHA or sensing and analyzing the current drawn by the rotary table. The former systems require the insertion of a potentially costly measurement sub and telemetering system which may be subject to failure along with the other tools in the BHA due to exposure to torsional vibrations. Furthermore, traditional telemetering systems do not render data readings on a real time basis. In addition, these measurement sub systems have traditionally relayed data to an analysis lab as opposed to the drill floor where the information is needed by the driller.
The other system used for automatically sensing and analyzing torsional vibrations in a drill string noted above combines a current sensor coupled to the power input to the rotary table with a computer program that analyzes the electrical signal produced by the current sensor. The program produces a plot of the standard deviation of the torque, (sigma torque), applied to the drill string as a function of time. In this system, a maximum per permissible standard deviation of the torque magnitude is specified based on empirical data. A rig floor alarm, in the form of a traffic signal light, is coupled to the system to warn the driller of potentially damaging high sigma torque situations. The system is configured so that an amber or red light will illuminate in the rig alarm whenever the maximum permissible standard deviation of the applied torque is exceeded.
Although the standard deviation sensing system eliminates the need for a costly and potentially failure-prone sensor sub, there are nevertheless disadvantages associated with such a system. The information displayed by the standard deviation system is, by and large, qualitative and requires human visual interpretation of the signal trace to determine whether a torsional vibration event is occurring.
The present invention is directed to overcoming, or at least minimizing, one or more of the foregoing disadvantages.